1. Field of the Invention
The present invention relates generally to the fields of molecular medicine and drug delivery and, more specifically, to a method of in vivo panning for identifying a molecule that homes to a specific organ.
2. Background Information
Although the effect of a particular pathology often is manifest throughout the body of the afflicted person, generally, the underlying pathology may affect only a single organ or tissue. In many cases, drugs are the treatment of choice for a patient suffering a particular disease. It is rare, however, that a drug will target only the diseased tissue or organ. More commonly, drug treatment results in undesirable side effects due, for example, to generalized toxic effects throughout the patient's body. The nausea, loss of hair and drop in blood count that occur as a result of treating of a cancer patient with chemotherapeutic agents are examples of the undesirable side effects that can occur due to drug treatment.
The undesirable side effects that can occur when drugs are used to treat a disease most often are due to the inability of the drug to specifically target the diseased organ or tissue. For example, a cancer chemotherapeutic agent that targets rapidly proliferating cells would be useful to kill rapidly dividing cancer cells. However, such an agent also kills normal proliferating hematopoietic and epithelial cells. Thus, the dose of such a drug that can be administered to a patient is limited due to its toxic effect on normal cells.
Efforts have been made to increase the target specificity of various drugs. In some cases, a particular cell type present in a diseased tissue or organ may express a unique cell surface marker. In such a case, an antibody can be raised against the unique cell surface marker and a drug can be linked to antibody. Upon administration of the drug/antibody complex to the patient, the binding of the antibody to the cell surface marker results in the delivery of a relatively high concentration of the drug to the diseased tissue or organ. Similar methods can be used where a particular cell type in the diseased organ expresses a unique cell surface receptor or a ligand for a particular receptor. In these cases, the drug can be linked to the specific ligand or to the receptor, respectively, thus providing a means to deliver a relatively high concentration of the drug to the diseased organ.
While linking a drug to a molecule that homes to a particular cell type present in a diseased organ or tissue provides significant advantages for treatment over the use of a drug, alone, use of this method is severely limited. In particular, very few cell type specific antibodies have been described and it can be difficult and time consuming to attempt to obtain an antibody that targets an organ in a particular patient suffering a pathology. Furthermore, few cell type specific surface markers have been described. Even where such markers have been described, the cells expressing the markers can be distributed among various tissues or organs, thereby limiting their usefulness as targets. Thus, it is important to identify specific target cell markers that are expressed in only one or a few tissues or organs and to identify molecules that specifically interact with such markers.
Various cell types can express unique markers and, therefore, provide potential targets for organ homing molecules. Endothelial cells, for example, which line the internal surfaces of blood vessels, can have distinct morphologies and biochemical markers in different tissues. The blood vessels of the lymphatic system, for example, express various adhesion proteins that serve to guide lymphocyte homing. For example, endothelial cells present in lymph nodes express a cell surface marker that is a ligand for L-selectin and endothelial cells in Peyer's patch venules express a ligand for the .alpha..sub.4 .beta..sub.7 integrin. These ligands are involved in specific lymphocyte homing to their respective lymphoid organs. Thus, linking a drug to L-selectin or to the .alpha..sub.4 .beta..sub.7 integrin may provide a means for targeting the drug to diseased lymph nodes or Peyer's patches, respectively, provided that these molecules do not bind to similar ligands present in a significant number of other organs.
Although the homing molecules present in the blood vessels of non-lymphoid tissues have not been clearly defined, the ability of lymphocytes to return to the organ in which they were first stimulated indicates that organ-specific endothelial markers exist. Similarly, the homing or metastasis of particular types of tumor cells to specific organs provides further evidence that organ-specific markers exist. However, there remains a need to identify other organ-specific cell markers and the molecules that bind to them.
Methods are now available for producing large populations of molecules. In addition, methods are available for screening libraries of molecules to identify those of interest. For example, phage peptide display libraries can be used to express large numbers of peptides that can be screened in vitro with a particular target molecule or a cell of interest in order to identify peptides that specifically bind the target molecule or the cell. Screening of such phage display libraries has been used, for example, to identify ligands that specifically bind various antibodies and cell surface receptors.
Screening of a phage display library generally involves in vitro panning of the library using a purified target molecule. Phage that bind the target molecule can be recovered, individual phage can be cloned and the peptide expressed by a cloned phage can be determined. Such a peptide can be useful for delivery of a drug linked to the peptide to cells expressing the target molecule.
Unfortunately, very few target molecules that are expressed by only one or a few cell types have been identified. Furthermore, even where such a target molecule is known, it is uncertain whether a peptide that specifically binds the target molecule, as determined using an in vitro panning method, will bind to the target molecule in vivo. As a result, the identification of a peptide from a phage display library using an in vitro panning method essentially represents only a starting point for determining whether the identified peptide can be useful for an in vivo procedure. Thus, a need exists to develop in vivo methods for screening large numbers of molecules such as peptides in order to identify those that can home to one or more selected organs. The present invention satisfies this need and provides related advantages as well.